The present invention relates to superconducting ultrabroadband antennas generally and, in particular, to a high-temperature superconductor, broadband self-limiting spiral antenna with a controllable signature. While the present invention will be discussed with reference to spiral antennas, it should be understood that the invention is applicable to other forms of transmission line antennas which are not, strictly speaking, spirals, such as log periodic, sinuous, deformed spiral, and ambidextrous antennas.
Broadband antennas are widely used in many contexts, including communications, as radar warning receivers, electronic support measures, and other civil and military applications. In those cases where a broad bandwidth is desired, it is usually necessary to sacrifice some other aspect of antenna performance in order to obtain the desired bandwidth. In some cases, it is necessary to make the antenna larger in order to increase bandwidth. In other cases, increased bandwidth is obtained at the expense of the radiation pattern of the antenna. Neither of these tradeoffs is especially desirable. Where it is desired to mount the antenna on an aircraft or other mobile platform, for example, increased size exacts a penalty in added weight and reduction in space available for other equipment. Degradation of the antenna""s radiation pattern can severely compromise antenna performance. Antenna designers are continually seeking ways to increase antenna bandwidth without sacrificing antenna performance and without incurring increases in size and weight.
Much attention has been devoted to improving the bandwidth of transmission line antennas, particularly to extending the bandwidth of spiral transmission line antennas into the VHF/UHF/SHF bands. Prior attempts have primarily focused on improving low-frequency response by increasing the size of the antenna. Increasing the diameter of the spiral, and concomitantly the length of the arms of the spiral, provides frequency scaling that always results in lower gain at the equivalent scaled Lower) frequency. As the length of the spiral arms (sometimes also called the xe2x80x9cwindingsxe2x80x9d) is increased, so is their resistance. In many cases, the efficiency of the antenna will become quite low because of losses due to the increased resistance. Thus, a wideband antenna will have lower efficiency at the lower end of the band, since attenuation from loss resistance is greater in that portion of the band. This is because there is a longer distance from the feed points to the xe2x80x9cone-wavelengthxe2x80x9d diameter radiating region of the spiral. The loss per unit length in the windings varies approximately as ƒxc2xd, but the length of windings which is effectively used to excite the xe2x80x9cone wavelengthxe2x80x9d diameter radiating region varies as 1/ƒ2. As a result, the loss variation due to this effect alone is an inverse function of frequency approximately equal to 1/ƒ{fraction (3/2)}.
The inventors have found that reducing conductor resistance by several orders of magnitude will change the conductor loss effect from a 1/ƒ{fraction (3/2)} relationship (high loss at low frequencies) back toward a ƒxc2xd relationship (low loss at low frequencies). The conductor resistance can be decreased dramatically by forming the spiral windings from a high-temperature superconducting (HTSC) material, i.e., materials which exhibit superconductivity at temperatures on the order of 77xc2x0 K. At 1.0 GHz, HTSC materials are expected to offer surface resistance values as low as 1 xcexcxcexa9 per square, which is several orders of magnitude below cryogenically-cooled copper at 77xc2x0 K.
The inventors are not aware of any previous attempts to improve antenna bandwidth using HTSC or other cryogenic conductors. While A. Septier and N. T. Viet have suggested using low temperature superconductors (which exhibit superconductivity while immersed in liquid helium at about 1.5xc2x0 K.) to improve antenna Q, it is noted that the result of such an improvement is a very narrowband antenna. See, A. Septier and N. T. Viet, xe2x80x9cMicrowave applications of superconducting materials,xe2x80x9d J. of Physics E., vol. 10, pp. 1193-1207 (1977). U.S. Statutory Invention Registration H653 discloses a superconducting superdirective antenna array using high temperature superconductors, but it discloses only a very narrow bandwidth.
There is a need for an ultrabroadband transmission line antenna (i.e., one having a low Q), which covers frequencies from the microwave region down to the VHF region of the spectrum. The present invention fulfills that need.
The present invention is broadly directed to a transmission line antenna assembly having a substantially continuous bandwidth from the microwave region of the electromagnetic spectrum to the VHF region of the spectrum. The antenna assembly comprises at least one balanced transmission line antenna element of high-temperature superconductor material (HTSC) supported by a substrate, an antenna cavity supporting the substrate and containing a thermally-conductive electromagnetic-energy-absorbing material therein, and a cryogenic cooler for cooling the antenna element to a temperature at which it exhibits superconductivity. Preferably, the substrate has a crystalline lattice compatible with that of the HTSC and is thermally matched to it.
More particularly, the present invention is directed to an antenna assembly having a plurality of antenna elements of high-temperature superconductor material supported by a substrate in which each element forms a spiral. Each element has a first end proximate the first end of each other element. The elements are interwound and define a concentric multi-arm spiral. The antenna assembly also comprises an antenna cavity supporting the substrate and containing a thermally-conductive electromagnetic-energy-absorbing material therein, and a cryogenic cooler for cooling the antenna elements to a temperature at which they exhibit superconductivity.